In communication systems having mobile subscriber units, the first step in obtaining service from the communication system is acquisition. Acquisition is the process of establishing a communication link between a communication station and a subscriber unit. Acquisition is necessary for registration of the subscriber unit, call set-up, answering call terminations, or to initiate services with a communication system.
In satellite communication systems using Time Division Multiple Access (TDMA) frequency channels, subscriber units must communicate at a proper frequency channel and time-slot. Failure to do so will result in interference with ongoing calls in adjacent time-slots and frequency channels and a loss of information.
In satellite-based communication systems where the communication stations are moving rapidly with respect to the subscriber units, the subscriber units must compensate for a wide range of propagation delays associated with the distance to a satellite from different points within the satellite's footprint. The subscriber units must also compensate for the frequency shift associated with the velocity of the satellite. This is especially true in satellite systems having low-earth orbits.
The use of random access channels is complicated by the geometry of a satellite system. The conventional method of collision avoidance on random access channels by carrier sensing is not effective in a satellite system because the subscriber units are often not in a radio line-of-sight of each other. Messages may be blocked by two or more subscriber units attempting to transmit to a satellite on the same channel at the same time.
The use of control channels in a mobile satellite system is also impaired because satellite channels are subject to momentary outages due to fading and blockage. These outages could cause the loss of important control messages and prevent completion of the acquisition process.
Thus what is needed are a method and apparatus for acquiring a traffic channel of a satellite communication system. What is also needed are a method and apparatus for synchronizing to a communication system's timing windows where the distances to the subscriber units are unknown. Further, what is also needed is a subscriber unit that sends signals in sync with the satellite's timing windows. What is also needed is a subscriber unit that corrects the frequency and timing of its transmitted signals to compensate for the velocity of a satellite. Further, what is also needed are a method and apparatus of communicating with a TDMA communication system without interfering with ongoing calls in adjacent time-slots and adjacent frequency channels. What is needed are a method and apparatus that achieve proper frame timing alignment, determine the correct downlink time-slot and correct for the Doppler-shift associated with satellite movement. What is also needed are a method and apparatus that perform these functions reliably in the presence of channel impairment such as multi-path fading or shadowing by obstructions. What is also needed are a method and apparatus that reduce collisions on acquisition channels of a communication system.